Amplifiers are devices that accept a varying input signal and produce an output signal that varies in the same way as the input, but with a larger amplitude. The input and output signals may consist of a current, a voltage, a mechanical motion, or any other signal. An electronic amplifier is a device for increasing the power of a signal. It does this by taking power from a power supply and shaping the output to match the input signal. This process invariably introduces some noise and distortion into the signal, and the process is not completely efficient. Amplifiers always produce some waste as heat.
Different designs of amplifiers are used for different types of applications and signals. Amplifiers broadly fall into three categories: small signal amplifiers, low frequency power amplifiers, and RF power amplifiers. The most common types of amplifiers are electronic and have transistors or electron tubes as their principal components. Electronic amplifiers are widely used in consumer electronic devices, such as in radio and television transmitters and receivers, as well as audio and stereo systems.
Amplifiers in their simplest form are built around a single transistor. In one type of single-transistor amplifier, known as a common-emitter circuit, a varying input voltage is fed to the base of the transistor, and the output appears at the transistor's collector; the ratio of the output voltage to the input voltage is called the voltage gain. For many purposes a single transistor does not provide sufficient gain, or amplification. In a cascade, or multistage, amplifier, the output of the first amplifying device (transistor) is fed as input to the second amplifying device, whose output is fed as input to the third, and so on until an adequate signal amplification has been achieved. In a device such as a radio receiver, several amplifiers boost a weak input signal until it is powerful enough to drive a speaker. Usually, multistage amplifiers are not made of discrete components, but are built as integrated circuits. Another less common group of electronic amplifiers use magnetic devices as their principal components.
Amplifier circuits are classified as A, B, AB, and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle is used to actually switch on the amplifying device. Class A amplifiers use all of the input signal. Class B amplifiers use half of the input signal. Class AB amplifiers use more than half of the input signal, but less than all of it. Class C amplifiers use less than half of the input signal.
Class A amplifiers are a fully linear amplifier with active circuit elements biased into their linear operating region. Class A amplifiers amplify over the whole of the input cycle. This means that the region must have enough voltage range to encompass the entire amplitude of an incoming signal in order to reproduce it without clipping or compressing at either extreme. They are the usual means of implementing small-signal amplifiers. They are not very efficient—a theoretical maximum of 50% efficiency is obtainable, but for small signals, this waste of power is small and tolerable. In a Class A circuit, the amplifying element is biased such that the device is always conducting to some extent, and is operated over the most linear portion of its characteristic curve (known as its transfer function or transconductance curve). Because the device is always conducting, even if there is no input at all, power is wasted. This is the reason for its inefficiency.
Class A designs are generally not preferred for audio power amplifiers, though some audiophiles believe that Class A gives the best sound quality due to its linear operation. In addition, some aficionados prefer vacuum tube designs over transistors, for a number of reasons. One is that the characteristic curve of a valve means that distortion tends to be in the form of even harmonics which, they claim, sound more “musical” than odd harmonics. Another is that valves use many more electrons at once than a transistor; thus statistical effects lead to a “smoother” approximation of the true waveform. Field-effect transistors have similar characteristics to valves, so these are found more often in high quality amplifiers than bipolar transistors. Historically, valve amplifiers often used a Class A power amplifier simply because valves are large and expensive; the Class A design uses only a single device. Transistors are much cheaper, so more elaborate designs that give greater efficiency but use more parts are still cost effective.
Class B amplifiers are somewhat more efficient than Class A amplifiers because they utilize two drive elements operating in a push/pull configuration. On the positive excursion of the signal, the upper element supplies power to the load while the lower is turned off. During negative excursions, the opposite operation occurs. This design increases operating efficiency, but suffers from the nonlinear turn-on, turn-off region created where the driver elements switch from their ON state to their OFF state. This switching error creates a condition commonly called cross-over distortion.
Class A/B: amplifiers remedy cross-over distortion to a great degree by combining the best features of both classes. The push/pull drivers are carefully biased just above their fully OFF state so that the transition between drivers is smoother. Therefore, each driver is never completely turned OFF. This alleviates most of the cross-over distortion at the expense of efficiency. An A/B amplifier is more efficient than a Class A amplifier.
Class C amplifiers are biased at or below cutoff. These amplifiers are often used for certain types of RF transmission, but are not commonly used in audio applications.
Amplifiers are an essential component in car audio applications. A stock car audio system refers to exactly what was specified by the manufacturer when the car was built. A custom car audio installation could mean anything from the upgrade of the radio to a full-blown customization of a car based around delivering exceptional sound quality or volume from audio equipment. The most common and familiar piece of audio equipment is the radio/tape player/CD player, which is generically described as a headunit. A recent development in headunit technology has been the addition of CD players with MP3 support.
High-end audio systems include component speakers that consist of a matched tweeter, mid-range and woofer set. These component pairs are available in two speaker and three speaker combinations, and include a cross-over which limits the frequency range that each component speaker must handle. In addition, a subwoofer(s) is provided for low frequency music information.
Amplifiers provide the necessary music power, measured in watts, to drive the speakers. High Power amplifiers require large gauge cable to provide adequate voltage and current to the amplifier. Alternators may be upgraded from the stock unit to increase the current capability of the vehicle's electrical system, often required of high-power amplifiers.
While the term car audio describes the sound system in an automobile, it also refers more broadly to the field of mobile entertainment and is becoming a sport at large. Many car audio enthusiasts enter their car audio systems into competitions, commonly known as “sound off” competitions. Organizations such as the International Auto Sound Challenge Association (IASCA) and the United States Autosound Competition International (USACI) sponsor and manage sound off competitions. There are two basic types of sound off competitions. One type is centered upon the Sound Quality (SQ) of a car audio system. The other type is based upon the Sound Pressure Level (SPL) of the car audio system.
In a sound pressure level competition, competitors are typically given 30 seconds in which to reach the maximum pressure level that their audio system can provide for a duration of around two seconds. This process of providing the maximum pressure level from the audio system for two seconds is referred to as “burping” the system.
Operating a high end car audio system in a competition, or for personal use, presents a variety of unique challenges. With a high end car audio system, an audio entusiast is trying to operate as much as a 10 kW amplifier off of a 12-volt car electrical system. Music has an average power level that is ⅛th its maximum root mean square power. As such, a vehicle's electrical system must have the capacity to provide eight times its average power output to meet the peak power needs. For a 10 kW class amplifier, this means that the amplifier will use thousands of amps of current from the battery-supported low voltage car electrical system at a peak music power level. In order to accommodate this level of current use, auto enthusiasts will typically over-build their car audio system. For sound off competitions, competitors may have ten or more car batteries consuming the entire space of the trunk and multiple alternators connected to the engine.
Not only do car audio enthusiasts have to overbuild their car electrical system to handle these peak music power demands, they must also overbuild the electrical system to handle the inherent inefficiencies of the electrical system. In an electrical system, power is equal to the resistance times the square of the current (P=ri2). When operating at an average music power level, a 10 kW system may use 100 amperes from the electrical system. At 100 amperes, a car electrical system having one milliohm of resistance would have a 10 W power loss. When operating at a peak music power level, a 10 kW system may use 1000 amperes from the electrical system. At 1000 amperes, a car electrical system having one milliohm of resistance would have a 1 kW power loss. This non-linear rise of power losses in relation to the current use forces one to operate an amplifier far below its rated capacity in a car electrical system.
There is therefore a great need to design an improved electrical system for car audio systems. There is a great need to provide a car audio system that can accommodate the peak power levels of music while still operating from a conventional car electrical system with few, if any, changes.